It is known to provide a spark ignition engine with a small ignition chamber adjacent to and connected with a main combustion chamber in which combustion of a charge, ignited by a spark plug in the ignition chamber, causes a jet of flame to shoot into the main chamber and ignite the main charge therein. It is also known to provide a lean air-fuel mixture to the main combustion chamber through a main inlet port and a rich air-fuel mixture to the auxiliary or ignition chamber through a second and smaller inlet port. In the usual case, separate intake valves are provided to control the admission of the lean and rich mixtures to the two chambers. This provision usually requires dual or interconnected actuating means for actuating the dual inlet valves.